Originally posted by Artman I know that invertor duty motors should be used when Variable Frequency Drives are used. What I don't know is why.

What does invertor duty do?

I think it has something to do with cleaning up the harmonic distortions caused by the drives. Is this it?

This is not a uniformly defined concept. Typically, it addresses the following:

Lower packing factors for the windings; to allow for more cooling.

Higher insulation ratings for the wire; for HV spikes caused by the PWM or PFM in the drive.

Greater cooling capacity.

Bearing shunts to prevent induced transient currents [again from the high frequency switching in the VFD] from passing through the bearing.

In addition to the electrical characteristics, the cooling is a major issue. Normally, a rule of thumb applies that we never run a motor at more than twice or less than half the rated value for sustained periods. VFDs allow much lower motor speed than was typically found in the past - before VFDs. So what happens is that people will try to run a motor a 5 or 10 Hz [~150 to 300 RPM], and the internal fan in the motor provides little to no cooling at these speeds. If we are under load at 10 Hz for example, the motor burns up.

Always check with the specific motor manufacturer to identify the lowest or highest rating for motor speed under a specified load.

Thanks Ivan. Everything you said makes sense. I will pass alkong the info to the guy who asked me (I told someone that VFDs required invertor duty rated motors and he said "Why?" I wasn't really sure, I just knew they did.)